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CSS

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HTML 

              
                <aside>Fixed child of <code>body</code></aside>

<main>
<h1>Testing a Root Container</h1>
<div class="controls">
  <fieldset>
    <legend>HTML element (root)</legend>
    <div>    
      <label for="root-container">
        container-type
      </label>
      <select name="root-container" id="root-container">
        <option value="revert" selected>normal</option>
        <option value="inline-size">inline-size</option>
        <option value="size">size</option>
      </select>
    </div>
    <div>    
      <label for="root-overflow">
        overflow
      </label>
      <select name="root-overflow" id="root-overflow">
        <option value="revert" selected>normal</option>
        <option value="auto">auto</option>
      </select>
    </div>
    <div>    
      <label for="root-height">
        height
      </label>
      <select name="root-height" id="root-height">
        <option value="revert" selected>normal</option>
        <option value="100">100%</option>
      </select>
    </div>
  </fieldset>
  <fieldset>
    <legend>BODY element</legend>
    <div>    
      <label for="body-overflow">
        overflow
      </label>
      <select name="body-overflow" id="body-overflow">
        <option value="revert" selected>normal</option>
        <option value="auto">auto</option>
      </select>
    </div>
    <div>    
      <label for="body-height">
        height
      </label>
      <select name="body-height" id="body-height">
        <option value="revert" selected>normal</option>
        <option value="100">100%</option>
      </select>
    </div>
  </fieldset>
</div>

<p>
  It doesn't matter 
  how we select the element in CSS --
  using <code>:root</code>
  or <code>html</code> --
  in either case we are selecting 
  the same element.
  On first glance,
  it seems like a perfect
  'container':
</p>

<ul>
  <li>
    We want it sized
    to the viewport,
    not the contents.
    The extrinsic size
    and expected overflow
    makes it a good candidate for
    two-axis
    <code>size</code> containment.
  </li>
  <li>
    Since it is a direct wrapper
    of a single <code>body</code> element,
    we would get both an outer container
    and an outer element responding to
    container queries --
    without any additional markup.
  </li>
</ul>

<p>
  However,
  this doesn't work as expected,
  and results in two common issues:
</p>
  
<ul>
  <li>
    Positioning of fixed elements
    relative to <code>html</code>
    rather than the viewport
  </li>
  <li>
    Scrollbars failing to propagate 
    to the viewport
  </li>
</ul>
  
<p>
  Play around with the toggles above
  to see how different types of containment,
  and different overflow/height settings
  on <code>html</code> and <code>body</code>
  interact.
</p>

<h2>Spacer Content</h2>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>
<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>

<p>
  Just an easy way to trigger overflow.
  Lorem ipsum dolor sit amet consectetur, adipisicing elit. Quibusdam velit, eligendi repudiandae sapiente sequi nobis, ratione tempore necessitatibus, est culpa maiores aliquid veritatis quos aperiam tempora asperiores hic! Id, debitis.
</p>
</main>
!

CSS 

              
                [data-container='size'] {
  container-type: size;
  --color: red;
}

[data-container='inline-size'] {
  container-type: inline-size;
  --stripe: orange;
}

[data-overflow='auto'] {
  overflow: auto;
}

[data-height='100'] {
  height: 100%;
}

aside {
  position: fixed;
  inset-block-start: 0.5em;
  inset-inline-end: 0.5em;
}

@layer defaults {
  * { box-sizing: border-box; }
  
  html {
    font-size: x-large;
    border: 0;
    background: repeating-linear-gradient(
        to bottom right,
        var(--color, var(--stripe, lightcyan)) 0% 1%,
        var(--color, lightcyan) 1% 2%
      ) repeat scroll;
  }
  
  body {
    margin: 0;
  }
  
  main {
    margin: 1em auto;
    background: white; 
    max-width: 60ch;
    padding: 1em;
  }
  
  h1 {
    margin-block-start: 0;
  }
  
  aside {
    padding: 1em;
    background: pink;
    border: medium double;
  }

  .controls {
    border-block: thin dotted;
    padding: 1em;
  }

  .help {
    font-size: smaller;
    margin: 0.5em 0 0;

    &::before {
      content: '(';
    }
    &::after {
      content: ')';
    }
  }
  
}
!

JS 

              
                const targets = {
  root: document.querySelector('html'),
  body: document.querySelector('body')
};

document.querySelectorAll('select').forEach((input) => {
  const inFor = input.id.split('-');
  const target = targets[inFor[0]];
  const attr = inFor[1];
  
  input.addEventListener('input', (e) => {
    target.setAttribute(`data-${attr}`, input.value);
  });
})
!
999px

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